1. Field of the Invention
The present invention relates to a magnet temperature estimating system for a synchronous electric motor.
2. Description of the Related Art
A permanent magnet synchronous electric motor is known in which permanent magnets are disposed in a rotor and the rotor is rotated by an interaction between the permanent magnets and a rotating magnetic field generated by a stator. When the temperature rises, the magnetic flux of each of the permanent magnets is reduced by a demagnetization phenomenon. Thus, the temperature of the permanent magnet needs to be controlled in such a way as to be suppressed to a level equal to or below an allowable temperature. To detect the temperature of the permanent magnet, a temperature sensor needs to be provided on the permanent magnet and this hinders the size reduction of the electric motor. In this regard, Patent Literature 1 (Japanese Patent Application No. 2007-6613) discloses a method of estimating the temperature of the permanent magnet without using a temperature sensor. In Patent Literature 1, the temperature of the permanent magnet is estimated from an induced voltage of an electric motor.
However, in the method disclosed in Patent Literature 1, a measurement error of the induced voltage increases in a low revolution number state in which the induced voltage is small, and an estimation accuracy for the temperature of the permanent magnet may become low.
The present invention has been made in view of the problem described above and an object thereof is to provide a magnet temperature estimating system for a synchronous electric motor which can improve the estimation accuracy for the temperature of the permanent magnet in a wide operation range from zero speed to a high number of revolutions of the electric motor.